1. Field of the Invention
The present invention generally relates to semiconductor devices. More particularly, the present invention relates to a semiconductor device which has employed an SOI structure, and is an effective technique for DSP, CPU and other semiconductor devices for use in portable equipment operable at a low voltage.
2. Related Arts
As disclosed in the Japanese Unexamined Publication Nos. 3-82151, 4-125960, 5-108194 or 6-21443, a technique which enables a semiconductor device to operate at a high speed by reducing a threshold voltage when the semiconductor device is in an operating state and to reduce a power consumption by increasing the threshold voltage when the semiconductor device is in a waiting (standby) state has been publicized. However, each of the semiconductor devices disclosed in these publications includes an external source circuit called a "charge pump" and controls the threshold voltage by applying a source voltage to a substrate or a well layer.
In a MOS transistor having a bulk structure, the substrate or well layer and source and drain regions disposed therein are separated from each other by a PN isolation region. In order to prevent an electric current from flowing when the PN isolation region takes the forward-bias direction, it is necessary to apply an electric potential higher than that of a ground potential or a source voltage on which the MOS transistor is operating to the substrate. To accomplish this purpose, a special circuit (external source circuit) is essential to the conventional technique.
However, the external source circuit poses problems because it not only requires an additional circuit area but also consumes additional electric power.